That being said, after having a political/ideological group I really liked removed off a major platform (that it followed the ToS of) without analogous opposite-side groups being removed, I'm at least very sympathetic to your position. I thing we both agree that harmless differences of opinion shouldn't be removed, but I'll agree to disagree on whether the law should be involved.
Well that's always the million-dollar question, isn't it? Take Twitter for instance. Whether there's an easy alternative to Twitter depends entirely on your reference class.
The social aspect makes this so much harder as people are unique. It's completely reasonable for someone to need to be on the same platform as some other particular person. Say, Elon Musk. Or pre-ban Trump if you want the least convenient example from a civics perspective. How do you justify kicking someone off a platform for having the wrong views if that's where their elected representatives spend most of their time?
But this perspective completely defangs your position in all but principle. Any privately-owned platform with important[1] people using it is basically any platform that has any reasonable amount of success. Even if you restrict to public officials, well, there's a lot of public officials.
[1] Note important is in the eye of the user, and people will generally self-select into platforms that have users that are important to themselves.
Just because some "famous" person uses a platform shouldn't mean the platform has to fall over itself to make sure their every word is hear and everyone can contact them.
Social media platforms are content-neutral. They want to be treated like the "public square" because a lot of benefits come along with that. But then at the same time, they censor content that they disagree with. That's where it crosses the line. If Twitter came out tomorrow and declared that they are officially a left-wing social network and updated their rules to reflect that, then I'd have no problem with them kicking every Republican off the platform and banning every person who complained about taxes being too high. But Twitter won't do that.
I don't want to get further off on the tangent about telecommunications infrastructure, so I'll just leave that alone.